Helminth (worm) parasites take an enormous toll on human health, especially in developing countries. Half a billion people suffer debilitating, sometimes fatal illness as a result of these infections, and subclinical effects of helminth parasitism include retarded physical and cognitive development. Despite their global health impact, and the pressing need for new treatments and preventative measures against them, there has been a decline in the numbers of investigators funded to work on these pathogens, and a consequent reduction in the number of young scientists choosing to enter this field. The paucity of modern molecular tools for interrogating gene function in parasitic helminths and the resulting degree to which helminth biology remains descriptive as opposed to mechanistic in nature are undoubtedly factors in this decline. Nevertheless, the free-living worm Caenorhabditis elegans and planaria can be studied using sophisticated molecular tools, suggesting that development of similar tools for parasitic worms is feasible. In this vein, we and others have had recent encouraging success in the experimental manipulation of gene expression in parasitic worms. Despite this, significant technical hurdles remain if molecular genetics is to become routine in helminth parasitology. Therefore, we propose to further develop and refine tools for the manipulation of gene expression in helminth parasites. We will focus on two important human pathogens, Strongyloides stercoralis and Schistosoma mansoni. These represent, respectively, the Nematoda and Platyhelminthes, two phyla of medically important helminths. The first of our two specific aims is to develop transgenic parasitic helminths that are amenable to experimentation. Work towards this aim will involve developing DNA constructs that allow regulated, tissue-specific transgene expression in transiently transformed S. mansoni and St. stercoralis. Also, we will seek to identify regulatory sequences that allow conditional transgene expression and both intra- and extracellular transport of recombinant proteins. We will also develop methods for establishing stably expressing transgenic lines through serial host passage. New methods for chromosomal integration of transgenes will be developed and insulator sequences will be identified as measures against epigenetic transgene silencing in both transiently and stably transformed worms. Under our second specific aim we will target expressed genes for silencing in S. mansoni and St. stercoralis. Hypotheses that the activities of RNAi processing enzymes homologous to C. elegans RDE-1, SAGO-1, SID-1 and SID-2 limit the silencing efficiency of exogenously applied dsRNA in St. stercoralis and that siRNAs administered to the host can silence genes of S. mansoni and St. stercoralis in vivo are to be tested under this aim. The proposed research has the potential to create new tools that will invigorate the molecular and cellular biological study of a neglected and important group of human pathogens. This application proposes to develop new methods to study gene function at the molecular level in Schistosoma mansoni and Strongyloides stercoralis, representatives of two major groups of parasitic helminths (worms). Its two specific aims propose to develop methods for DNA transformation of S. mansoni and St. stercoralis with transgenes capable of regulated tissue specific expression and of stable expression through serial host passage, and for targeted silencing or disruption of gene expression in the two parasites. Because reliable techniques of this nature are either underdeveloped or altogether unavailable, the proposed project has the potential to invigorate the field of parasitic helminthology, which is currently perceived as largely descriptive, as opposed to mechanistic, in nature.